1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery. More specifically, the invention relates to a nonaqueous electrolyte secondary battery in which a separator includes a heat resistance layer.
2. Description of Related Art
A nonaqueous electrolyte secondary battery such as a lithium ion battery or a nickel metal hydride battery is preferably used as a portable power supply or a vehicle-mounted power supply with high output because it is light-weight and has high energy density. Among these nonaqueous electrolyte secondary batteries, particularly, in a high-capacity secondary battery (for example, a vehicle-mounted battery) in which high-rate charging and discharging (rapid charging and discharging) is repeated, not only superior battery performance but also a high level of safety are required.
In a general configuration of this nonaqueous electrolyte secondary battery, a separator is provided between a positive electrode and a negative electrode to insulate both the electrodes. This separator is typically formed of a resin microporous sheet and can allow charge carriers to be held and move between the positive and negative electrodes. In addition, this separator has a shutdown function of blocking the movement of charge carriers when a battery is at a high temperature. In addition to the shutdown function, this separator is required to prevent the internal short-circuit of the positive and negative electrodes and to suppress a further increase in the temperature of the entire battery even when the separator is thermally shrunk or broken due to overheating during an abnormal situation.
As means for satisfying the above-described requirements, a separator that includes a porous heat resistance layer (HRL) on a surface of a resin porous sheet is disclosed (for example, refer to Japanese Patent Application Publication No. 2012-195224 (JP 2012-195224 A) and Patent Application Publication No. 2013-105521 (JP 2013-105521 A)). An HRL typically contains particles of an inorganic compound (inorganic filler) as a major component and has high heat resistance and insulating properties (non-conductivity). In addition, Japanese Patent Application Publication No. 2013-115031 (JP 2013-115031 A) discloses a water-soluble salt such as a lithium (Li) salt, a sodium (Na) salt, a potassium (K) salt, or an ammonium (NH3) salt of carboxymethyl cellulose (CMC) that is contained as a thickener in a separator coating liquid for forming such an HRL.
However, in this type of nonaqueous electrolyte secondary battery, the capacity slowly decreases along with repeated charging-discharging cycles. A nonaqueous electrolyte secondary battery including the above-described HRL-provided separator has a problem in that the above tendency is significant, that is, cycle characteristics are poor.